1. Field of the Invention
This invention relates to a process for treating water vapor which is generated while concentrating an aqueous urea solution containing small amounts of ammonia and carbon dioxide and, more particularly, it relates to a process for recovering ammonia and carbon dioxide from water vapor which is generated while concentrating an aqueous urea solution containing small amounts of said ammonia and carbon dioxide.
2. Description of the Prior Art
The production of crystal urea or molten urea from carbon dioxide and ammonia is carried out by a process which comprises reacting carbon dioxide and ammonia under urea-forming temperatures and pressures, passing the resultant urea synthesis effluent containing urea, unreacted ammonium carbamate and water through a plurality of stages for decomposing the unreacted ammonium carbamate (such as two stages composed of one high pressure stage and one low pressure stage; three stages composed of two high pressure and one low pressure stages, or stripping by carbon dioxide or ammonia under a pressure substantially equal to a urea synthesis pressure and subsequent high and/or low pressure decomposition stage; it being general to provide a flash separation stage under an almost normal pressure or a vacuum subsequent to a low pressure decomposition stage) wherein pressure is reduced by steps in order to separate the unreacted ammonium carbamate from the reaction mixture by decomposition, and subjecting the resultant aqueous urea solution containing small amounts of ammonia and carbon dioxide to concentration under normal pressure or vacuum to separate urea as crystals, or alternatively, concentrating the urea aqueous solution containing small amounts of ammonia and carbon dioxide into substantially water-free molten urea without yielding crystal urea.
In this process, water vapor generated from a concentrator generally contains small amounts of aqueous urea solution mist, ammonia and carbon dioxide, so that discharge of the water vapor by condensing the vapor into water results not only in water pollution, but also in loss of urea and ammonia. Accordingly, it is common practice to condense the water vapor by indirectly cooling into a dilute aqueous solution of urea, ammonia and carbon dioxide, and to subject the dilute aqueous solution to rectification for separation of a gaseous mixture of ammonia, carbon dioxide and water vapor from the solution, the gaseous mixture being recovered by absorption together with an off-gas from the low pressure decomposition stage of unreacted ammonium carbamate. However, the water vapor in the gaseous mixture is large in amount, so that the absorbate obtained by the absorption of the gaseous mixture disadvantageously tends to be diluted to excess. Further, additional cooling water will be inconveniently required for removal of heat of condensation of the water vapor.
The following two processes are considered to be available for the recovery of ammonia and carbon dioxide from the water vapor discharged from the step of concentration of the aqueous urea solution for reuse in urea synthesis. The first process includes condensing the water vapor to form a dilute aqueous ammonium carbamate solution and subjecting the aqueous solution to decomposition together with urea synthesis effluent containing unreacted ammonium carbamate under high or low pressure to decompose the ammonium carbamate contained in the dilute aqueous ammonium carbamate solution as well as unreacted ammonium carbamate.
In this process, however, all of the water vapor from the concentration step of the aqueous urea solution is fed to the decomposition step and the water vapor from the concentration step gradually increases in amount, the aqeous urea solution being finally excessively diluted. In order to avoid the excessive dilution, it is required either to discard part of the water vapor from the system or to provide an additional system for recovering ammonia and carbon dioxide from the water vapor.
In the second process, the water vapor fed from the concentration step is compressed and subjected to low pressure decomposition for condensation of most of the water vapor and for collection of ammonia and carbon dioxide involved as well as ammonia and carbon dioxide which are formed in decomposition of unreacted ammonium carbamate. However, this process has the same disadvantage as the first process. Moreover, ammonia and carbon dioxide are also unavoidably condensed together with part of water vapor in compressing the gaseous mixture. Accordingly, it is necessary to prevent ammonia and carbon dioxide from being condensed, requiring a complicated apparatus.
It is therefore an object of the present invention to provide an improved process for the recovery of ammonia and carbon dioxide from water vapor generated in concentrating an aqueous urea solution.